Nanofibers ranging from 10 nm to 1000 nm have been used in filtration medium to capture submicron particles below 1000 nm. The ability of nanofibers to capture particles is believed to be due to combination of interception of submicron particles by the fibers as well as the Brownian motion or “random walk” of submicron particles, both of which facilitate the particles to be captured by the large surface/mass ratio of the nanofibers.
Conventional filtration media may have a layer of nanofibers with a size distribution in a range of 100 to 300 nm laid on a substrate layer of the medium, as depicted in FIG. 1A. Further increase of capture capability may be obtained by increasing the nanofiber surface area such as by reducing the fiber diameter and/or by increasing the packing density of the nanofibers, as measured in terms of grams of nanofibers per square meter. However, since the nanofibers do not possess structural rigidity, when more fibers are laid, the additional nanofibers often compress to form a dense (i.e. low porosity or void space) and thick layer that substantially increase the pressure drop across the medium. This lowers the permeability of the filtration medium to airflow, and resulted in undesirable breathability to the filtration medium.
Consequently, it is desirable to develop an improved nanofiber filtration medium that has a high filtration efficiency but low pressure drop. It is also desirable to develop a method of making the nanofiber filtration medium having these improved properties.